1. Field of the Invention
The invention relates to a chemical-mechanical polishing (CMP) method applied in shallow trench isolation (STI), and more particular, to a chemical-mechanical polishing method incorporated with a partial reverse active mask applied in shallow trench isolation.
2. Description of the Related Art
For a very large scale integration (VLSI) or even an ultra large scale integration (ULSI), chemical-mechanical polishing is the only technique that provides global planaration. Since this technique greatly reduces feature size of an integrated circuit, the manufacturers rely on this technique for planarization process. A great interest to further develop this technique is evoked for further reduction in feature size and fabrication cost.
As the dimension of semiconductor devices becomes smaller and smaller, deep sub-half micron technique, for example, a line width of 0.25 .mu.m, or even 0.18 .mu.m, is used. To planarize the wafer surface by chemical-mechanical polishing, especially to planarize the oxide layer within in a trench, becomes more and more important. To prevent the formation of a recess on the surface of the oxide layer within a shallow trench isolation of a larger area, a reverse tone active mask is used in process. An etch back process is also performed to obtain a better chemical-mechanical polishing uniformity. However, a misalignment often occurs.
In a conventional process of forming a shallow trench isolation, since the active regions have different dimensions, the dimensions of shallow trench between active regions are different. In FIG. 1A to FIG. 1E, a cross sectional view of the process for forming a shallow trench isolation by chemical-mechanical polishing is shown. In FIG. 1A, a pad oxide layer 15 and a silicon nitride layer 16 are formed on a substrate 10. Using photolithography and anisotropic etching, a shallow trench 14 and an active region 12 are formed. The dimensions of the shallow trench 14 are various according to the various dimensions of the active region 12.
In FIG. 1B, using atmosphere pressure chemical vapor deposition (APCVD), an oxide layer 18 is formed over the substrate 10 and fills the shallow trench 14. Due to the topography of the shallow trench 14 within the substrate 10 and the characteristics of step coverage of the oxide layer 18, the surface of the deposited oxide layer 18 is undulating but smooth. A photo-resist agent is coated on the oxide layer 18. Using photolithography, a reverse tone active mask 20 is formed. The reverse tone active mask 20 covers the surface of the shallow trench 14 and becomes complementary to the active regions 20. It is known that during the formation of the reverse tone mask 20, a misalignment often occurs. Consequently, the reverse tone active mask 20 covers a range of the oxide layer 18 beyond the shallow trench 14.
In FIG. 1C, the exposed part of the oxide layer 18, that is, the part which is not covered by the oxide layer 18, is etched away until the silicon nitride layer 16 is exposed. The resultant structure of the oxide layer is denoted as 18a. As shown in the figure, the oxide layer 18a covers most of the shallow trench 14 and a small part of the silicon nitride layer 16 on the active region. In FIG. 1D, the reverse tone active mask 20 is removed. It is found that a recess 22 is formed since the oxide layer 18a does not covered the shallow trench 14 completely.
In FIG. 1E, the oxide layer 18a is polished by chemical-mechanical polishing until the oxide layer 18a has a same level as the silicon nitride layer 16. Since the oxide layer 18a formed by APCVD has a smooth profile, so that it is difficult to be planarized. In addition, it is obvious that the recess 22 is formed since the oxide layer 18a does not fill the shallow trench 14 completely. A kink effect is thus easily occurs by the recess 22. That is, a current leakage or a short circuit is caused. The yield of the wafer is affected.